Tenthredo retura Ross is here placed in synonymy under T. maxima (Norton) as the subspecies T. maxima retura Ross (syn.nov., stat.nov.). The range of T. maxima is associated with montane meadows or semi-open forest habitats in the montane zone of the Rockies, the Cascades, and the Sierra Nevada. Following the analysis of 24 characters from 2405 specimens, we recognized 12 geographically distinct populations, clustered into two subspecies, T. m. maxima and T. m. retura. Originally included in the originalis group of the genus Tenthredo, T. maxima is characterized as a group of its own with no close relatives in North America. We describe the species and its subspecies, give their distribution, discuss their geographical variation, and attempt to reconstruct their zoogeographical history during the Pleistocene.